Fracture of a viscous liquid

When a viscous liquid hits a pool of liquid of same nature, the impact region is hollowed by the shock. Its bottom becomes extremely sharp if increasing the impact velocity, and we report that the curvature at that place increases exponentially with the flow velocity, in agreement with a theory by Jeong and Moffatt. Such a law defines a characteristic velocity for the collapse of the tip, which explains both the cusp-like shape of this region, and the instability of the cusp if increasing (slightly) the impact velocity. Then, a film of the upper phase is entrained inside the pool. We characterize the critical velocity of entrainment of this phase and compare our results with recent predictions by Eggers

Temporal Dissection of Stimulus-Driven and Task-Driven Processes during Perceptual Decision about 3D SFM Stimuli

International audienceAnalyzing the visual properties of a stimulus, such as its shape or its motion, involves a temporally con-strained cascade of processes including sensory integration, attentional selection and perceptual decision. In natural condi-tions, these processing stages may temporally overlap. In this study, we used a time constrained paradigm designed to segre-gate these stages, and recorded MEG activity to characterize their cortical correlates. The experimental sequence comprised a structure-from-motion (SFM) target flanked by pre and post masks, which limited the stimulus-driven processes in time. The MEG responses were recorded in three perceptual tasks bearing on the presence or absence of the target SFM stimulus, on its direction of motion or on its 3D shape. Subjects' responses were delayed to the end of the trials us-ing a random stimulus-response mapping. The analysis of the evoked magnetic fields reveals temporal-ly non-overlapping responses for stimulus-driven and task-related processes. Source reconstruction reveals that the occipital, dorsal and ventral stimulus-related areas follow different temporal pro-files, The response in IPS slowly rising after the beginning of the activity in the lateral occipital cortex and falling back to the baseline prior the delayed motor response, may reflect accumulating evidence on the presented stimulus leading to a motor response, in line with the results from single cell studies in monkey LIP [1]

Impaired saccadic eye movement in primary open-angle glaucoma

PURPOSE: Our study aimed at investigating the extent to which saccadic eye movements are disrupted in patients with primary open-angle glaucoma (POAG). This approach followed upon the discovery of differences in the eye-movement behavior of POAG patients during the exploration of complex visual scenes. METHODS: The eye movements of 8 POAG patients and 4 healthy age-matched controls were recorded. Four of the patients had documented visual field scotoma, and 4 had no identifiable scotoma on visual field testing. The eye movements were monitored as the observers watched static and kinetic targets. The gain, latency, and velocity-peak latency of the saccades recorded were then analyzed. RESULTS: In POAG patients, with abnormal visual fields, watching a static target, the saccades were delayed and their accuracy was reduced, compared with those of normal observers. In POAG patients, with normal and abnormal visual fields, watching a kinetic target, a task involving precise motion analysis, the latency and accuracy of the saccades were impaired, compared with those of normal observers. CONCLUSIONS: Our findings suggest that POAG alters saccade programming and execution particularly in the case of moving targets

Gravitational oscillations of a liquid column

We report gravity oscillations of a liquid column partially immersed in a bath of liquid. We stress in particular some peculiarities of this system, namely (i) the fact that the mass of this oscillator constantly changes with time; (ii) the singular character of the beginning of the rise, for which the mass of the oscillator is zero; (iii) the sources of dissipation in this system, which is found to be dominated at low viscosity by the entrance (or exit) effects, leading to a long-range damping of the oscillations. We conclude with some qualitative description of a second-order phenomenon, namely the eruption of a jet at the beginning of the rise.Comment: 22 pages, pdf. Submitted to Physics of Fluid

Permeability of a bubble assembly: From the very dry to the wet limit

We measure the permeability of a fluidized bed of monodispersed bubbles with soap solution characteristic of mobile and non-mobile interfaces. These experimental data extend the permeability curves previously published for foam in the dry limit. In the wet limit, these data join the permeability curves of a hard sphere suspension at porosity equal to 0.4 and 0.6 in the cases of mobile and non-mobile interfaces respectively. We show that the model of permeability proposed by Kozeny and Carman and originally validated for packed beds of spheres (with porosity around 0.4) can be successfully applied with no adjustable parameters to liquid fractions from 0.001 up to 0.85 for systems made of monodisperse and deformable entities with non-mobile interfaces

Mechanical tuning of the evaporation rate of liquid on crossed fibers

We investigate experimentally the drying of a small volume of perfectly wetting liquid on two crossed fibers. We characterize the drying dynamics for the three liquid morphologies that are encountered in this geometry: drop, column and a mixed morphology, in which a drop and a column coexist. For each morphology, we rationalize our findings with theoretical models that capture the drying kinetics. We find that the evaporation rate depends significantly on the liquid morphology and that the drying of liquid column is faster than the evaporation of the drop and the mixed morphology for a given liquid volume. Finally, we illustrate that shearing a network of fibers reduces the angle between them, changes the morphology towards the column state, and so enhances the drying rate of a volatile liquid deposited on it

Bifurcation study of a neural field competition model with an application to perceptual switching in motion integration.

Perceptual multistability is a phenomenon in which alternate interpretations of a fixed stimulus are perceived intermittently. Although correlates between activity in specific cortical areas and perception have been found, the complex patterns of activity and the underlying mechanisms that gate multistable perception are little understood. Here, we present a neural field competition model in which competing states are represented in a continuous feature space. Bifurcation analysis is used to describe the different types of complex spatio-temporal dynamics produced by the model in terms of several parameters and for different inputs. The dynamics of the model was then compared to human perception investigated psychophysically during long presentations of an ambiguous, multistable motion pattern known as the barberpole illusion. In order to do this, the model is operated in a parameter range where known physiological response properties are reproduced whilst also working close to bifurcation. The model accounts for characteristic behaviour from the psychophysical experiments in terms of the type of switching observed and changes in the rate of switching with respect to contrast. In this way, the modelling study sheds light on the underlying mechanisms that drive perceptual switching in different contrast regimes. The general approach presented is applicable to a broad range of perceptual competition problems in which spatial interactions play a role

Combining Feature Selection and Integration—A Neural Model for MT Motion Selectivity

Background: The computation of pattern motion in visual area MT based on motion input from area V1 has been investigated in many experiments and models attempting to replicate the main mechanisms. Two different core conceptual approaches were developed to explain the findings. In integrationist models the key mechanism to achieve pattern selectivity is the nonlinear integration of V1 motion activity. In contrast, selectionist models focus on the motion computation at positions with 2D features. Methodology/Principal Findings: Recent experiments revealed that neither of the two concepts alone is sufficient to explain all experimental data and that most of the existing models cannot account for the complex behaviour found. MT pattern selectivity changes over time for stimuli like type II plaids from vector average to the direction computed with an intersection of constraint rule or by feature tracking. Also, the spatial arrangement of the stimulus within the receptive field of a MT cell plays a crucial role. We propose a recurrent neural model showing how feature integration and selection can be combined into one common architecture to explain these findings. The key features of the model are the computation of 1D and 2D motion in model area V1 subpopulations that are integrated in model MT cells using feedforward and feedback processing. Our results are also in line with findings concerning the solution of the aperture problem. Conclusions/Significance: We propose a new neural model for MT pattern computation and motion disambiguation that i